Remnants of a late Paleocene through middle Eocene depositional system are preserved in the stratigraphic record on San Miguel and Santa Cruz Islands and in coastal San Diego.
On San Miguel Island, upper Paleocene starved-basin mudstones were deposited on Maestrichtian middle submarine fan sandstones. Lying conformably on the Paleocene deposits is a lower Eocene sequence of starved-basin to fan-fringe mudstones and Poway rhyolite-bearing, middle submarine-fan depositional lobes of conglomerate. These facies are in turn overlain by an upper lower Eocene through lower middle Eocene retrogradational sequence of shale-filled channels, levees, fan-fringe, and starved-basin deposits. The remainder of the middle Eocene strata are braided, middle submarine-fan sandstones and mudstones.
Paleogene sedimentation on Santa Cruz Island began in the late Paleocene and continued uninterrupted through the entire Eocene Epoch. Upper Paleocene strata are composed of detritus washed from paralic environments to the east-northeast. These inner shelf deposits form a sequence of sublittoral sheet sandstones which coarsen upward into an interval of Poway rhyolite-bearing channelized conglomerate. Lower Eocene outer shelf mudstones overlie the conglomerate and the remainder of the depositional facies record sedimentation under progressively deepening marine conditions. These environments include passive slope, fan-fringe, and inner submarine-fan channels containing Poway rhyolite-bearing conglomerate.
In San Diego, the Mount Soledad Formation is composed of six sedimentary facies: (1) paralic (upper estuarine), (2) deltaic, (3) alluvial fan and fluvial channels, (4) submarine canyon head, (5) inner fan channel, and (6) slope. Poway rhyolite clasts are found in facies 2 through 5.
Based primarily on lithostratigraphic correlations, deltaic facies of the Mount Soledad Formation are proximal equivalents to the sublittoral sand sheet facies on Santa Cruz Island. Alluvial fan, fluvial channel, submarine canyon head, and inner submarine-fan conglomerate portions of the Mount Soledad sequence are equivalent to lower Eocene middle submarine-fan conglomerates on San Miguel Island.
The Paleogene sequence of facies on San Miguel and Santa Cruz Islands are equated lithostratigraphically to equivalent facies of the Mount Soledad Formation. Comparison of the changes in depths of deposition of the vertical sequence of facies with Tertiary eustatic changes suggests that the succession of marine facies at all these localities developed synchronously in response to changes in global sea level.